1. Technical Field of the Invention
The present invention relates to spread spectrum communications systems and, in particular, to cell search activities performed by a mobile station to acquire time synchronization with a base station and obtain the cell-specific long code used in a spread spectrum communications system.
2. Description of Related Art
The cellular telephone industry has made phenomenal strides in commercial operations throughout the world. Growth in major metropolitan areas has far exceeded expectations and is outstripping system capacity. If this trend continues, the effects of rapid growth will soon reach even the smallest markets. The predominant problem with respect to continued growth is that the customer base is expanding while the amount of electromagnetic spectrum allocated to cellular service providers for use in carrying radio frequency communications remains limited. Innovative solutions are required to meet these increasing capacity needs in the limited available spectrum as well as to maintain high quality service and avoid rising prices.
Currently, channel access is primarily achieved using Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) methods. In frequency division multiple access systems, a physical communication channel comprises a single radio frequency band into which the transmission power of a signal is concentrated. In time division multiple access systems, a physical communications channel comprises a time slot in a periodic train of time intervals over the same radio frequency. Although satisfactory performance is being obtained from FDMA and TDMA communications systems, channel congestion due to increasing customer demand commonly occurs. Accordingly, alternate channel access methods are now being proposed, considered and implemented.
Spread spectrum comprises a communications technique that is finding commercial application as a new channel access method in wireless communications. Spread spectrum systems have been around since the days of World War II. Early applications were predominantly military oriented (relating to smart jamming and radar). However, there is an increasing interest today in using spread spectrum systems in communications applications, including digital cellular radio, land mobile radio, and indoor/outdoor personal communication networks.
Spread spectrum operates quite differently from conventional TDMA and FDMA communications systems. In a direct sequence code division multiple access (DS-CDMA) spread spectrum transmitter, for example, a digital symbol stream for a given dedicated or common channel at a basic symbol rate is spread to a chip rate. This spreading operation involves applying a channel unique spreading code (sometimes referred to as a signature sequence) to the symbol stream that increases its rate (bandwidth) while adding redundancy. Typically, the digital symbol stream is multiplied by the unique digital code during spreading. The intermediate signal comprising the resulting data sequences (chips) is then added to other similarly processed (i.e., spread) intermediate signals relating to other channels. A base station unique scrambling code (often referred to as the "long code" since it is in most cases longer than the spreading code) is then applied to the summed intermediate signals to generate an output signal for multi-channel transmission over a communications medium. The dedicated/common channel related intermediate signals advantageously then share one transmission communications frequency, with the multiple signals appearing to be located on top of each other in both the frequency domain and the time domain. Because the applied spreading codes are channel unique, however, each intermediate signal transmitted over the shared communications frequency is similarly unique, and through the application of proper processing techniques at the receiver may be distinguished from others.
In the DS-CDMA spread spectrum mobile station (receiver), the received signals are recovered by applying (i.e., multiplying, or matching) the appropriate scrambling and spreading codes to despread, or remove the coding from the desired transmitted signal and return to the basic symbol rate. Where the spreading code is applied to other transmitted and received intermediate signals, however, only noise is produced. The despreading operation thus effectively comprises a correlation process comparing the received signal with the appropriate digital code to recover the desired information from the channel.
Before any radio frequency communications or information transfer between a base station and a mobile station of the spread spectrum communications system can occur, the mobile station must find and synchronize itself to the timing reference of that base station. This process is commonly referred to in the art as "cell searching". In a direct sequence code division multiple access spread spectrum communications system, for example, the mobile station must find downlink chip boundaries, symbol boundaries and frame boundaries of this timing reference clock. The most common solution implemented to this synchronization problem has the base station periodically transmit (with a repetition period T.sub.p), and the mobile station detect and process, a recognizable pilot code c.sub.p of length N.sub.p chips as shown in FIG. 1. The pilot code may also be referred to in the art as a spreading code for long code masked symbols. This pilot code is sent with a known modulation and without any long code scrambling. In one type of CDMA communications system, each base station utilizes a different, known pilot code taken from a set of available pilot codes. In another type of CDMA communications system, all base stations utilize the same pilot code, with differences between base stations being identified through the use of differing phase shift of the pilot code for the transmissions.
In the spread spectrum receiver of the mobile station, the received signals are demodulated and applied to a filter matched to the pilot code(s). It is, of course, understood that alternate detection schemes, such as sliding correlation, may be used for pilot code processing. The output of the matched filter peaks at times corresponding to the reception times of the periodically transmitted pilot code. Due to the effects of multi-path propagation, several peaks may be detected relating to a single pilot code transmission. From processing these received peaks in a known manner, a timing reference with respect to the transmitting base station may be found with an ambiguity equal to the repetition period T.sub.p. If the repetition period equals the frame length, then this timing reference may be used to synchronize mobile station and base station communications operation with respect to frame timing.
While any length of N.sub.p in chips for the transmitted pilot code c.sub.p may be selected, as a practical matter the length of N.sub.p in chips is limited by the complexity of the matched filter implemented in the mobile station receiver. At the same time, it is desirable to limit the instantaneous peak power P.sub.p of the pilot code signal/channel transmissions in order not to cause high instantaneous interference with other spread spectrum transmitted signals/channels. To obtain sufficient average power with respect to pilot code transmissions given a certain chip length N.sub.p, it may become necessary in the CDMA communications system to utilize a pilot code repetition period T.sub.p that is shorter than a frame length T.sub.f as illustrated in FIG. 2.
Another reason for transmitting multiple pilot codes c.sub.p within a single frame length T.sub.f is to support inter-frequency downlink synchronization in the compressed mode known to those skilled in the art. With compressed mode processing, downlink synchronization on a given carrier frequency is carried out during only part of a frame rather than during (across) the entire frame. It is possible, then, with only one pilot code c.sub.p per frame, that compressed mode processing could miss over a significant time period detecting the pilot code completely. By transmitting multiple pilot codes c.sub.p during each frame, multiple opportunities per frame are given for compressed mode processing detection, and at least one pilot code transmission will be capable of being detected.
There is, however, a drawback with respect to reception and synchronization experienced with multiple pilot code c.sub.p transmission within a single frame length T.sub.f. Again, the received signals are demodulated and applied to a filter (or correlator) matched to the known pilot code. The output of the matched filter peaks at times corresponding to the reception times of the periodically transmitted pilot code. From processing these peaks, a timing reference for the transmitting base station relating to the pilot code repetition period T.sub.p may be found in the manner well known in the art. However, this timing reference is ambiguous with respect to the frame timing and thus does not present sufficient information to enable base/mobile station frame synchronization to the timing reference. By ambiguous it is meant that the boundary of the frame (i.e., its synchronization) cannot be identified from the detected pilot code peaks alone.
The process for cell searching may further involve obtaining the cell specific long code used on the downlink to scramble downlink dedicated and common channel communications. The dedicated channels comprise both traffic and control channels, and the common channels also comprise traffic and control channels (which may include the broadcast control channel (BCCH)). A long code group code c.sub.lci is preferably transmitted synchronously with (and further preferably orthogonal to) the pilot codes c.sub.p as illustrated in FIG. 3. This long code group code is sent with a known modulation and without any long code scrambling. Each long code group code c.sub.lci indicates the particular subset of a total set of long codes to which the cell specific long code utilized for the transmission belongs. For example, there may be one-hundred twenty-eight total long codes grouped into four subsets of thirty-two codes each. By identifying the transmitted long code group code c.sub.lci, the receiver may narrow its long code acquisition search in this example to only the thirty-two long codes contained in the subset identified by the received long code group code c.sub.lci.
Frame timing information may be found from a combined processing of the received pilot codes c.sub.p and long code group codes c.sub.lci. A mobile station first identifies pilot code timing by applying a c.sub.p matched filter to a received signal and identifying peaks. From these peaks, a timing reference with respect to the slots may be found. Although ambiguous as to frame timing, the determined slot locations identify the timing for the simultaneous transmission of the long code group code c.sub.lci. Correlation is then performed at the known slot locations to obtain the long code group code c.sub.lci identification. From this identification, the number of possible cell specific long codes used for the transmission is reduced. Lastly, a correlation is performed against each of the reduced number of long codes (i.e., those long codes contained in the c.sub.lci identified subset) at each of the known slots to determine which cell specific long code is being used for the transmission and provide a phase shift reference. Once the phase shift is found, frame timing is identified.
In connection with the transmission of multiple pilot codes c.sub.p within a single frame length T.sub.f, the determination of frame timing is alternatively assisted in the manner disclosed in U.S. application for patent Ser. No. 08/884,002, entitled "MOBILE STATION SYNCHRONIZATION WITHIN A SPREAD SPECTRUM COMMUNICATIONS SYSTEM", filed Jun. 27, 1997, by having each of the slots include not only a pilot code c.sub.p, as in FIG. 2 described above, but also a framing synchronization code c.sub.s transmitted with a known modulation and without long code scrambling, as illustrated in FIG. 4. The pilot code is the same in each slot and across the repeating frames. The framing synchronization codes, however, are unique for each slot in a frame, and are repeated in each frame.
To obtain frame timing information, a mobile station first identifies pilot code timing by applying a c.sub.p -matched filter to a received signal and identifying peaks. From these peaks, a timing reference with respect to the slots may be found. While this timing reference is ambiguous as to frame timing, knowledge of the slot locations indirectly points to the location of the framing synchronization code c.sub.s within each located slot. The mobile station then further correlates the set of known framing synchronization codes c.sub.s to the received signal at the locations of framing synchronization codes. Given that the position of each framing synchronization code c.sub.s relative to the frame boundary is known, once a correlation match is found at the location, the boundary of the frame relative thereto (and hence, the frame timing) is then also known.
Although the foregoing methods for obtaining synchronization information provide satisfactory results, their efficiency leaves much to be desired. For example, the processing of the long code group code c.sub.lci does not directly provide a frame timing indication, thus requiring additional correlations to be performed at each identified slot location to determine frame synchronization. Conversely, while the processing of the framing synchronization code c.sub.s provides a frame timing indication, completion of the cell searching process still further requires the performance of additional correlations to determine the cell specific long code being used for transmission. In each case, the additional correlations being performed occupy valuable processing resources, are complex to implement, and slow the cell searching process. There is a need then for a more efficient method of obtaining both a frame timing indication and a long code indication during the cell searching process.